Average resources per site?

Obviously every site/blog is different but I was wondering, for future growth analysis, how much space on average do you calculate per blog you host. I'm allowing up to a gig of space per site but know that is way more than most will use. I'm trying to figure out when I'll need to move from my VPS to a dedicated box. Could someone hosting 500+ active blogs please advise what they are seeing in way of space and bandwidth? Thanks!